I have the same issue, started about 5 years ago. My A/C doesn't work either, cause of a leak, but I have to remove the dash to fix that. As for no heat, I have replaced the water pump, thermostat, all hoses and flushed the systems forward and backward. I'd have to remove the dash to replace the heater core. People tell me there is a "heat box" under the dash that opens to allow heat to come through. I do get a small amount of heat, but not enough when it is really cold. The engine heats up just fine. The Haynes manual doesn't mention a heater box.

Do you feel a temp. difference on the return side of the heater core? or are both hoses hot. (HOT CAREFUL) If both side are about the same then i would check the blend door and the vacume to the blend door valve. HHMM this year may have an electric blend door motor. Does every thing else on in the heating / ac controls seem to work like switching from defrost-vent-floor?

Had similiar issue after the loser tranny mechanics R&R'd tranny and they broke the vacuum line on the passenger's side near the tranny dipstick. Fixed the busted line and it all worked. What was happening was it wouldn't switch between defrost to dash to floor. I can't recall if it was messed up changing temps but I do know it wouldn't change air direction. Good luck, let us know what you find out.

As for no heat, I have replaced the water pump, thermostat, all hoses and flushed the systems forward and backward. I'd have to remove the dash to replace the heater core. People tell me there is a "heat box" under the dash that opens to allow heat to come through. I do get a small amount of heat, but not enough when it is really cold.

What it looks like when your instrument panel and steering column are out:

That big black plastic dude with the ducts that runs from the transmission well to just about the cowl on the passenger side is the HVAC enclosure. If you look closely you can see the copper pipes leading from the top of the heater core into the engine compartment.

It's a big job, but not one that requires all that much mechanical savvy. Just follow the procedure in the 2001 factory service manual that's available in the forum FAQ/DIY section, but instead of disconnecting the electrical connection for the blend door motor, you remove the cable that operates the blend door. One metal clip at the end of the cable just slides right up and off of the blend door actuator shaft, and one plastic doohickey that ties the sheath to the HVAC enclosure, which also just slides up and out -- though it might require a little gentle persuasion after 14 years in place. It's said that you can remove that by going over the top of the instrument panel when it's rocked rearward, but I removed the passenger airbag and reached right in through the hole where it lives.

Then it's just a matter of removing the HVAC enclosure, pulling the heater core out, doing whatever needs doing for your A/C, popping the new heater core in, and "assembly is the opposite of removal". You'll probably want to have on hand four of those plastic snap-in nuts that hold the dash to the firewall up at the top because your old ones are likely cracked... at the dealership, part number 06504485AB, probably $6.25 each.

And if you're sporting the factory original dash, it's probably not going to survive that operation. About $250 will bring you a new one, delivered, from LMC Truck.

So I have been searching, found this thread. The cable behind the heater/ac switch has a nice kink in it, preventing the heater from being switched on without being inadvertently sprung back to the cool air option.